An automobile manual will provide you with a list of numbers about your car or motorbike. One of them is the compression ratio. In the case of of my KZ 305 LTD, that number is 9.5.
The number that my compression tester gives me is 145 psi in each cylinder of the motorbike engine. Uh derp, what do I do with that?
Well, it turns out that atmospheric pressure is about 15 psi. 145 psi divided by 15 psi makes 9.6666. Rad! That means the engine works!